Nanotube device and method of fabrication

ABSTRACT

A nanotube device and a method of depositing nanotubes for device fabrication are disclosed. The method relates to electrophoretic deposition of nanotubes, and allows a control of the number of deposited nanotubes and positioning within a defined region.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with U.S. Government support under grantcontract number AFOSR Grant: FA9550-05-1-0461 awarded by the Air ForceOffice of Scientific Research. The Government has certain rights in thisinvention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to commonly owned U.S.patent application Ser. No. 11/765,788, now U.S. Pat. No. 7,736,979,entitled “Method of Forming Nanotube Vertical Field Effect Transistor,”filed concurrently herewith, which is herein incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention generally relates to a nanotube device and methodof forming the device, and more particularly, to a method forcontrollably depositing one or more nanotubes in a defined region.

BACKGROUND OF THE INVENTION

There are many applications where a nanotube, e.g., a carbon nanotube(CNT), or an array of nanotubes, can be employed as a sensing or activedevice element in an electrical probe or electronic device. In theseapplications, electrical contact must be made with the nanotube, whichrequires accurate positioning of the nanotube with respect to variousconductive links (i.e. interconnects) and other circuitry.

Aside from the need for precise alignment, properties of the nanotubealso need to be controlled in order to provide device performanceaccording to desired specifications. For example, many transistorapplications for CNTs are best achieved with single wall carbonnanotubes (SWNT) rather than multi-wall carbon nanotubes (MWNT).Furthermore, as an active element of a transistor, a semiconductingSWNT, rather than a metallic SWNT, is required. For other applicationssuch as interconnects and nanoprobes, however, a metallic CNT ispreferred.

Existing fabrication methods for CNT devices do not fully address bothneeds for alignment and property control. In addition, in CNT electricaldevice fabrication, at least one interconnect level may be processedbefore CNT deposition. The most common metallization schemes, e.g., withaluminum and copper interconnects, often impose thermal budgetconstraints for subsequent processing steps. Chemical vapor deposition(CVD) methods, which are typically used for depositing CNTs, are notcompatible with aluminum or copper interconnects because of therelatively high temperatures involved.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a nanotube device and amethod of depositing nanotubes for device fabrication.

One embodiment relates to a method that includes: (a) defining a regionon a structure by an aperture, (b) configuring the aperture to control anumber of nanotubes to be deposited by electrophoresis in the region,and (c) depositing the number of nanotubes in the region byelectrophoresis.

Another embodiment of the invention provides a nanotube-based sensor,that includes an insulating layer over a metal contact formed on asubstrate, an aperture formed in the insulating layer, the apertureextending to the metal contact and defining a region at the metalcontact, a carbon nanotube disposed inside the aperture and having afirst end contacting the region at substantially a center of the region,and a second end coupled to a molecule having a functional group forinteracting with a sample.

Yet another embodiment provides a method of forming a carbonnanotube-based device, the method includes providing a substrate with aninsulating layer formed on a metal contact, forming an aperture throughthe insulating layer to expose a region of the metal layer, immersingthe substrate in an electrolytic fluid containing carbon nanotubes,providing a metal electrode in the electrolytic fluid, applying a biasvoltage across the metal contact and the metal electrode, and depositingat least one carbon nanotube in a substantially perpendicularorientation with respect to the region, in which one end of the carbonnanotube contacts the region proximate a center of the region.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is schematic cross-sectional view of a nanotube-based structurethat can be fabricated using embodiments of the present invention;

FIGS. 2A-E are schematic diagrams illustrating an experimental setup anda process sequence for depositing a carbon nanotube according to oneembodiment of the present invention;

FIGS. 3A-C are schematic illustrations of the electric fielddistributions around an aperture with a diameter of 100 nm and a depthof 50 nm;

FIGS. 4A-B are schematic illustrations of electric field distributionsaround an aperture with a diameter of 500 nm and a depth of 50 nm;

FIG. 5A is a schematic illustration of a nanotube sensor array;

FIG. 5B is a cross-sectional view of the nanotube sensor array of FIG.5A;

FIGS. 6A-J are illustrations of cross-sectional views of structuresduring various stages of a sensor fabrication sequence;

FIG. 7 is a schematic illustration of an experimental arrangement ofusing a nanotube sensor as an intracellular probe;

FIG. 8 is a schematic illustration of a nanotube-based transistor thatcan be fabricated using embodiments of the present invention; and

FIGS. 9A-B are schematic illustrations of a configuration of an aperturesuitable for implementing embodiments of the present invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

In the fabrication of CNT devices, there is often a need to provide avertically oriented CNT inside an aperture. In transistor fabricationprocesses, depending on the specific stage or levels, the aperture isalso referred to as a via.

Embodiments of the present invention provide a method of depositingnanotubes in a region defined by an aperture, with control over thenumber of nanotubes to be deposited, as well as the pattern and spacingof nanotubes. Specifically, electrophoretic deposition, along withproper configuration of the aperture, allows at least one nanotube to bedeposited in a target region with nanometer scale precision. Pre-sortingof nanotubes, e.g., according to their geometries or other properties,may be used in conjunction with embodiments of the invention tofacilitate fabrication of devices with specific performancerequirements.

FIG. 1A is a schematic cross-sectional view of a nanotube structure 100that can be fabricated using embodiments of the present invention. Thestructure 100 includes a substrate 102, over which an insulatingmaterial layer 104 has been deposited. The insulating layer 104 has beenpatterned to form an aperture 106, which exposes a top surface 108 ofthe substrate 102. A single CNT 110 is deposited inside the aperture 106so that one end 112 of the CNT 110 contacts the top surface of thesubstrate 102. The substrate 102 is a conducting material such as ametal or a conducting film (deposited over an insulating material) thatallows a bias voltage to be applied for electrophoretic deposition ofthe nanotube 110.

Embodiments of the present invention allow the CNT 110 to be depositedinside the aperture 106, to the exclusion of other CNTs. The aperture106, which has to be sufficiently large to accommodate the CNT 110, maybe patterned using different lithographic processes. Thus, in oneembodiment, the aperture 106 may have a diameter (D) ranging from aboutthe lower limit (e.g., resolution) of the lithography process to about100 nm. For example, existing lithography at 193 nm readily provides aresolution limit of about 90 nm. In one embodiment, the substrate 102has a lateral dimension (e.g., extending across the aperture)sufficiently large to meet level-to-level overlay constraints withrespect to the aperture 106. As will be shown below, the CNT 110 can bedeposited proximate the center of the aperture 106, e.g., with a lateralalignment precision of a few nanometers. Furthermore, the CNT 110 may bepre-selected to have a preferred physical property including multiwallCNT versus single wall CNT and or conducting CNT versus semiconductingCNT.

FIGS. 2A-D illustrate schematically an experimental setup forelectrophoresis and a sequence for depositing a CNT on a substrateaccording to one embodiment of the present invention. Electrophoreticdeposition (EPD) is driven by the motion of charged particles, dispersedin a suitable solvent, towards an electrode under the influence of anelectric field. Particles less than about 30 μm size can be used insuspensions with low solid loading and low viscosity. In general,whether nanotubes are deposited in the form of bundles or individualtubes depends on the nature of the suspension and the relativemobilities of each, which depends on their shapes and the associatedresistance to diffuse towards the contact surface inside the aperturesor vias.

FIG. 2A shows a substrate structure 200 with a conductive layer 202. Aninsulating layer 204 is provided over the conductive layer 202, and oneor more apertures 206 are patterned in the insulating layer 204. Thesubstrate structure 200 is immersed in a liquid bath 220, e.g., at roomtemperature, containing an electrolyte and a suspension of CNTs 210 in asuitable solvent.

Successful EPD requires preparation of a stable dispersion. In general,an electrostatically stabilized dispersion can be obtained withparticles of high ζ-potential, while keeping the ionic conductivity ofthe suspensions low. SWNTs have shown high ζ-potential values at low pHvalues. It is also known that the presence of charging salts can play animportant role in improving adhesion of the nanotubes to substrates andincreasing the deposition rates.

In one embodiment, 10 mg of purified SWNTs are suspended in 30 ml ofdistilled water, and 10⁻⁴ moles of magnesium nitrate hexahydrate[Mg(NO₃)26H₂0] is added to the suspension and sonicated for about 2-3hrs. In general, it is preferable that the nanotubes in the liquid bath220 be pre-sorted for the type of nanotubes according to applicationneeds. For example, while semiconducting SWNTs are used as activeelements in transistors, either semiconducting or metallic nanotubes maybe used for probes or other devices. A few drops of non-ionic Triton-Xsurfactant are added to improve the suspension with a final pH ofsolution at about 4.

Aside from hydrogen ions (H⁺), shown as circles in FIG. 2A, the liquidbath 220 also contains magnesium ions, Mg²⁺, which tend to adsorb orattach to the CNTs. An electrode 224, e.g., a platinum electrode, isimmersed in the liquid bath 220 and connected to a positive terminal ofa DC voltage source 222. The conducting layer 202 is connected to aswitch 226.

In FIG. 2A, when the switch 226 is open and there is no current flowinside the liquid bath 220 (current flow may be measured using anammeter A), the CNTs are randomly distributed in the suspension and anydeposition on the substrate will be random.

In FIG. 2B, the switch 226 is closed, thus connecting the conductivelayer 202 to a negative terminal of the DC source 222. With a DCpotential, e.g., in a range of about 5V-25V, applied across the platinumelectrode 224 and the conductive layer 202, charged particles or speciesin the fluid will move towards either the cathode or the anode. Forexample, H⁺ ions and positively charged CNTs will move towards thesubstrate structure 200, which is the cathode in this case.

Since H⁺ ions have higher mobility than other positively chargedspecies, including the CNTs, H⁺ ions will arrive at the substratestructure 200 faster than other charged species, and thus,preferentially accumulate on the surface of the insulating layer 204, asshown in FIG. 2B. The positively charged surfaces of the insulatinglayer 204 result in an electric field being produced around eachaperture 206.

Positively-charged CNTs arriving near the substrate structure 200 aredirected by the electric field towards the center of each aperture 206,as shown in FIG. 2C. Details regarding this “focusing” effect will bepresented in later discussions. In one embodiment, the apertures 206 andelectric field distribution are configured so that only one CNT (shownas CNT 210*) is deposited inside each aperture 206, even though thediameter (or lateral dimension) of the aperture 206 is large enough tophysically accommodate additional CNTs. The CNT 210* is disposed insideeach aperture 206 in a “longitudinal” manner, i.e., the length of theCNT 210* is along the same direction as the depth of the aperture 206,with one end of the CNT contacting the conductive layer 202.

FIG. 2D shows that the unattached end of CNT 210* tends to align orpoint towards the platinum electrode, and further, serves as a focalpoint for additional CNTs. Thus, a second CNT 210A becomes attached tothe free end of CNT 210*, e.g., in a lengthwise manner, with additionalCNTs attaching to each other end-to-end. The substrate structure 200 isthen removed from the bath 220, washed in distilled and de-ionizedwater, and dried with an inert gas. After drying, only the CNTs 210*that are attached to the conductive layer 202 remain, and the resultingstructure, such as one illustrated in FIG. 2E, is ready for furtherprocessing.

Since different devices often require different properties of thenanotubes for proper operation and/or optimum performance, it may beadvantageous to provide a pre-sorting of the nanotubes prior toelectrophoretic deposition. For example, nanotubes may be sortedaccording to their properties such as semiconducting versus metallic,single-walled versus multi-walled, or they may be sorted according togeometries or dimensions such as lengths, diameters, and so on.

Since different types of nanotubes have different mobilities, e.g.,longer or multiwalled nanotubes will generally have lower mobilitycompared to shorter or single-walled nanotubes, electrophoresis may alsobe used for sorting purposes. Such sorting can be done prior to theelectrophoretic deposition so that the nanotubes in the bath have arelatively uniform distribution in terms of properties and/orgeometries. Alternatively, if the nanotubes in the electrophoresis bathhave a relatively wide distribution in terms of geometries or otherproperties, a certain degree of sorting may also be achieved “in situ”during deposition by virtue of the different mobilities of thenanotubes.

The degree of focusing that directs the nanotubes towards the apertureis affected by the magnitude and shape of the electric fielddistribution, along with the configuration of the aperture. To providecontrol over the number of deposited nanotubes as well as theirpositioning, a finite element model is used to investigate the electricfield distribution as a function of various input parameters. Parametersor factors that are relevant for controlling nanotube deposition includethe aperture configuration, nanotube properties, characteristics of theinsulating layer and substrate, bias potential, dielectric properties ofthe solution, among others. The aperture configuration may generallyinclude the shape, dimensions (e.g., width, length, depth, ratios ofdimensions), sidewall profile, and so on. The nanotube properties maygenerally include the dimensions (e.g., length, diameter), single-walledor multi-walled, semiconducting or metallic.

The electric field around the aperture results from a combination of thepotential applied to the metal layer on the substrate structure andcharges that accumulate on the surface of the insulating layer. Thepositive charge accumulation on the dielectric layer covering thecathode creates an electric field that opposes the field arising fromthe bias applied between the anode and cathode. Once the two electricfields become equal and opposite, positive charges will no longer beattracted to the surface of the insulating layer. This “saturationcharge density”, σ, which determines the strength of the nanoscopic lensfrom the resulting electric field distribution, can be calculated from:σ=∈₀∈_(r)E  Eq. (1)where E is the magnitude of the electric field between the anode andcathode, ∈₀ is the permittivity of free space, and ∈_(r) is the relativepermittivity of the liquid.

As an example, for E=10³ V/m, ∈₀=8.85×10⁻¹² Farad/meter and the liquidis water ∈_(r)=80, the surface charge density σ is equal to 7.1×10⁻⁷Coulomb/meter².

Once the specific aperture geometry is selected and the surface chargedensity is calculated, the electric field in the region near theapertures and the motion of positively charged particles can becalculated using finite element analysis techniques that are well known.Thus, with proper configuration and design, one can obtain an electricfield distribution to produce a desired focusing or lens effect todirect the nanotube deposition.

FIGS. 3A-C show the results of electric field distributions around anaperture 306 having a diameter of 100 nm and a depth of 50 nm. In thisexample, a negative 10V bias is applied to the conductive layer 302.FIG. 3A shows the electric field distribution before H⁺ ions areaccumulated on the insulating surface. The electric field distributionis relatively uniform, with field lines mostly perpendicular to thesurfaces of the insulating layer 304. As shown in the figure, the fieldline directions are indicated by arrows pointing towards a region ofnegative potential. Only slight deviations of the field lines are seenat or close to the aperture 306.

FIG. 3B shows the modified electric field distribution after the surfaceof the insulating layer 304 is saturated with H⁺ ions. The arrows 320above the insulating layer 304 show that positively charged species willbe repelled away from the surface, while arrows 322 on either side ofthe aperture 306 show that the field lines are directed inwards, i.e.,towards an area above the aperture 306. Near the center of the aperture306, the field lines are directed downwards, i.e., towards the interiorof the aperture 306, as indicated by arrows 334. Thus, positivelycharged species such as CNTs are directed towards the aperture 306.

After sufficient charges have accumulated to reach the charge saturationpoint, the electrostatic lens effect will direct all charged particlestowards the center of the aperture 306. The equipotential lines for thisgeometry favor the focusing of mobile charged nanotubes towards thecenter of the aperture 306. In this case, the diameter of the aperture306 is 100 nm and the depth is 50 nm. In this example, since theelectric field distribution around the aperture 306 is substantiallysymmetric with respect to a central longitudinal axis of the aperture,the CNT 310 is also substantially centered inside the aperture 306.Thus, one end of the CNT 310 is attached to a region of the conductivelayer 302 defined by the aperture 306 (i.e., the exposed region at thebottom of the aperture), e.g., within a few nanometers of the center ofthe defined region.

FIG. 3C shows the electric field distribution after one CNT 310 has beendeposited inside the aperture 306. Since the CNT is conductive and is inelectrical contact with the conductive layer 302, the electric fielddistribution is modified by the deposited CNT 310. Furthermore, if theaperture 306 is sufficiently small, as it is in this case, the electricfield lines tend to concentrate towards the free end of CNT 310, insteadof directing towards the interior of the aperture 306. Thus, the freeend of CNT 310 becomes a focal point for further deposition ofnanotubes, instead of being deposited at the bottom of the aperture 306.

In general, for a fixed potential difference between the referenceelectrode and the metal contact at the bottom of the aperture, thestrength of the focusing effect is inversely proportional to thediameter of the aperture for a fixed aperture depth.

FIG. 4A-B show different results obtained for an aperture 406 having adiameter of 500 nm and a depth of 50 nm, with a negative 10V biasapplied to a conductive layer 402. FIG. 4A shows the electric fieldlines around aperture 406, with H⁺ ions accumulated on the surface ofinsulating layer 404, and FIG. 4B shows the electric field linesmodified by a CNT 410 inside the aperture 406. In this case, the CNT 410is positioned with a lateral offset from the center 406C of the aperture406, which may arise, for example, from a random direction of approachin the bath, followed by the electric field directing the nanotube toits location of deposition. As suggested by the field lines in thefigure, more than one CNT may be deposited inside the aperture 406.

In this case, the electric field distribution will not provide apreferential direction to guide the nanotubes towards the center regionof the aperture 406. The final location of the nanotube will depend onthe initial position of the nanotube before the bias is applied. For alarge aperture, e.g., diameter or lateral dimension of greater thanabout 100 nm, the unattached end of the first deposited nanotube maystill be the focal point for further nanotube deposition. However, whenthe lateral dimension of the aperture is sufficiently large, theelectric field will also direct other nanotubes to other locations onthe exposed surface of the conductive layer 402.

Although results suggest that an aperture diameter of about 100 nmprovide a transition or reference point below which deposition isrestricted to a single nanotube, while apertures larger than about 100nm tend to favor deposition of more than one nanotubes, it is understoodthat this reference point may vary with specific combinations ofnanotubes and/or structural configurations.

Aside from the aperture diameter (or lateral dimension), otherparameters, e.g., shape, aspect ratio (defined as depth or height ofaperture divided by lateral dimension), among others, may also be usedfor the purpose of controlling deposition of nanotubes, for example, byproviding different configurations according to the nanotube propertiesand/or geometries.

Results of another finite element analysis also show that, for nanotubeswith a 10 nm diameter and a length of 100 nm, and an aperture formed insilicon nitride with a diameter of 100 nm and a depth (or height) oflarger than 18 nm, only one nanotube will be deposited inside theaperture. This suggests that an aperture with an aspect ratio of atleast 0.18 or greater may be used to restrict the number of depositednanotubes to only one. For a nanotube with a smaller diameter, a largeraspect ratio may be required in order to restrict the deposition to onlyone nanotube. Similar analysis can be used to simulate probablelocations of deposited nanotubes for other aperture configurations andnanotube properties. While a two dimensional analysis is suitable forsituations in which a plane of symmetry is available, a threedimensional analysis can generally be used for other situations. Thus,finite element analysis can be used for nanoscopic lens design as aguide to providing nanotube deposition with additional levels ofcontrol.

Many different nanotube-based devices may be fabricated using the methodof the present invention. While the method can generally be applied tothe deposition of nanotubes within apertures of different dimensions, itis particularly well-suited for situations in which it is desirable tocontrol the number of nanotubes to be deposited or the lateralpositioning or alignment of the nanotube. Examples of nanotube-baseddevices that can benefit from this method include vertical CNTtransistors, chemical sensors or biosensors, among others.

FIG. 5A is a schematic illustration of a nanotube-based sensor arraydevice 500 according to one embodiment of the present invention, andFIG. 5B is a cross-sectional view along a vertical plane containing lineBB′. Device 500 generally includes one or more nanotubes 510 that aredeposited over a substrate 501. A conductive material 502, e.g., ametal, is formed at selected regions of the substrate 501 to provideconductive paths to the nanotubes 510. Nanotubes 510 can be depositedonto the conductive material 502 using electrophoresis as previouslydescribed, e.g., by forming apertures in an insulating layer 504provided over the conductive material 502, and applying a bias voltageto the conductive material 502. A sheath 512 is also formed around eachnanotube 510 to provide passivation and insulation for the nanotube 510.By insulating the sidewalls of the nanotubes 510 and leaving only thetips exposed, potential background noise may be reduced, thus increasingthe electrical sensitivity of the sensor. The interconnects to thenanotubes 510 are used for measuring changes in the CNT electricalcharacteristics.

A device similar to that shown in FIGS. 5A-B may be used forinvestigating intracellular activities in biological cells.Specifically, a CNT is a good candidate for such a probe because itssmall diameter (e.g., compared to cell membrane thickness) can minimizedistortions to the cell that is being investigated with the probe.

Depending on the specific sensor applications, different functionalmolecules 514 are provided to the other end of the nanotube 510. Ingeneral, SWNTs are preferred for sensor applications, although there maybe situations in which MWNTs may also be used.

FIGS. 6A-J are schematic cross-sectional views of a sensor devicestructure during various stages of a process sequence suitable forforming a device array such as that shown in FIG. 5A. FIG. 6A shows aconductive portion 602 formed on a substrate 600. For a biosensor,quartz may be used as the substrate material, which will facilitate theviewing of a biological sample with an optical microscope intransmission mode. In one embodiment, quartz wafers of 100 mm diameterand 350 μm thickness may be used, and they can be cleaned and preparedprior to device fabrication using methods known to one skilled in theart. In general, however, any suitable substrate will suffice, includingsilicon. If the starting substrate is silicon, or other conductingmaterial, then an insulating layer will first have to be deposited priorto the metal interconnects.

The conductive portion 602, e.g., an interconnect metal, can bedeposited and patterned using photolithography and resist liftofftechniques known to one skilled in the art. The interconnect metal needsto be suitable for maintaining electrical contact and adhesion withCNTs, and may include, but not limited to, cobalt (Co), nickel (Ni), oriron (Fe).

To facilitate adhesion between the metal interconnect to the quartzsubstrate, an adhesion layer (not shown) may also be formed prior to theformation of the conductive portion 602. In one embodiment, cobalt (Co)is used as a metal interconnect, and a 20 nm thick chromium (Cr)adhesion layer is used to promote adhesion of cobalt to the quartzsurface. The Cr layer may be evaporated at a rate of about 2 Ångstromsper second (Å/s), and a 120 nm thick Co layer can then be evaporated ata rate of about 1 Å/s. A thickness of about 20 nm and 120 nm may be usedfor the Cr and Co layers, respectively. The cobalt metal interconnectcan also serve as the cathode during electrophoretic deposition of thenanotube.

In the embodiment where CNTs are deposited using electrophoresis, theconductive portion 602 is configured to be electrically connected tocontact pads (not shown) provided at the edge or periphery of thesubstrate 602. In one embodiment, each conductive portion 602 upon whicha CNT will be deposited is provided as part of a continuous conductivelayer formed over the substrate 600, in order to simplify the electricalconnection paths. This facilitates electrical grounding of the metalduring electron-beam lithography (if e-beam is used during fabrication)and provides a single connection point for electrophoretic deposition ofCNTs. In one example, electrical connections between different devicesare made in the kerf (area between each device), which allows theconnections to be broken by a dicing saw when the substrate is cut tofacilitate assembling the devices. Alternative configurations may alsobe used for providing the electrical connections needed duringfabrication, and a variety of conventional lithographic and etchingprocesses may be adapted for this purpose.

For each individual sensor device, a metal contact is needed to provideelectrical connections to external circuits, for example, by solderingor wire bonding. In one embodiment, gold (Au) is used as the metalcontact material. The metal contact can be formed using photolithographyand resist liftoff techniques. FIG. 6B shows a structure during themetal contact formation stage, in which a photoresist layer 603 has beenpatterned to form an aperture 608 extending to an underlying region ofthe conductive portion 602. A conductive layer 604 is deposited over thepatterned resist 603 and the exposed region of the conductive portion602. When the resist layer 603 is removed from the structure of FIG. 6B,only the conductive layer inside the aperture 606 remains, resulting ina structure shown in FIG. 6C.

FIG. 6D shows an insulating layer 608 deposited over the substrate 600and the conductive portion 602 and subsequently patterned to form anaperture 610 (also referred to as vias or windows). In one embodiment,silicon nitride (SiNx) is used as the insulating layer 608. Thethickness of the insulating layer 608 is selected to provide anappropriate aspect ratio for electrostatic lens implementation afterpatterning. For example, a 50 nm thick SiNx film is suitable for formingapertures with a lateral dimension, e.g., diameter, of about 100 nm. Forexample, a 50 nm thick low stress SiNx film may be deposited at lowtemperatures by plasma enhanced chemical vapor deposition (PECVD) usingPlasma Therm 790 standard equipment at a temperature of about 350° C. Inthe context of a sensor device, the SiNx layer serves two functions.Aside from providing an insulation layer for forming the aperturesduring device fabrication, the SiNx layer, in the completed sensor, alsoprovides an insulating barrier between the conducting interconnect metaland the cell culture solutions in which measurements will be performed.

Using a suitable lithography process, aperture 610 having a lateraldimension of about 100 nm or less can be formed in the insulating layer608. The aperture 610 is sufficiently large to accommodate a nanotube tobe deposited onto substrate 602. In one embodiment, the aperture 610 hasa diameter ranging from about the lower limit (e.g., resolution) of thelithography process to about 100 nm. In one embodiment, opticallithography using 193 nm source illumination may be used to pattern theaperture in photoresist, providing a resolution of about 90 nm.Alternatively, these apertures may also be fabricated usingelectron-beam lithography or a focused ion beam milling technique.Apertures with dimensions of less than about 100 nm are suitable forproviding the electrostatic lens effect during electrophoreticdeposition of nanotubes. The lithography technique of the interconnectmetal and vias will limit the separation between the nanotube devices.

The structure of FIG. 6D can then be immersed in an electrolytic bathfor electrophoretic deposition of nanotubes, using a similar setup aspreviously discussed in connection with FIG. 2. In one embodiment, apatterned structure with a cobalt metal layer on a quartz substrate isused as the negative electrode, and a platinum wire is used as thepositive electrode. DC bias voltages in the range of about 5V-25V may beused.

FIG. 6E shows a structure after deposition of a nanotube 650 inside theaperture 610. Prior to electrophoretic deposition, the nanotubes may bepresorted for metallic SWNTs and filtered to limit bundles of SWNTs. Thelength of the CNTs or SWNTs may, for example, be less than about 1micron (μm), and will typically be determined by the application needs.In the case of a biosensor such as an intracellular probe, the lengthmay depend on requirements for mechanical stability and penetrationdistance into the target cell.

After the nanotube 650 is deposited in the aperture 610, its verticalorientation may be affected by the rinse process or by charging effects.The nanotubes can be re-aligned to the vertical direction by applying anelectrical potential between the metal level on which it was depositedand a metal plate above the wafer substrate. This may be done, forexample, in a reactor prior to the subsequent deposition process in thefabrication sequence. Plasma processing systems typically have a metalplate above the wafer that is part of the electrical circuit forgenerating the plasma. By establishing a DC or AC electric field betweenthis metal plate (or another electrode) and the metal level that thenanotube is deposited on, the nanotube can be re-aligned to a desiredorientation prior to subsequent processing.

After nanotube deposition, a conformal film 612 of an insulatingmaterial having a thickness range of about 2-5 nm may be formed toencapsulate and passivate (or insulate) the nanotube 650. Suitablematerials for this encapsulation film include SiNx or suitable polymers,e.g., polytetrafluoroethylene.

Since the sorting of nanotubes before or during electrophoreticdeposition usually does not provide sufficiently precise control of thelengths of the nanotubes, additional trimming may be needed in order toprovide a nanotube with a certain length specification. This can beachieved by the process steps illustrated in FIGS. 6G-H, showing apolysilicon layer 613 formed (e.g., grown or deposited) over theencapsulation film 612 (FIG. 6G). The polysilicon layer 615 can then bepolished back, e.g., using chemical mechanical polishing (CMP), untilthe desired nanotube length is achieved, as shown in FIG. 6H.

A portion of the encapsulation film 612 around the tip of the depositednanotube 650 can then be removed by a brief reactive ion etching (RIE)or chemical etch to uncover the tip of the nanotube 650. The length ofnanotube 650 that is uncovered will depend on the etch rate and the timeduration of the etch. RIE etches for SiNx are standard processes forfabrication of complementary metal oxide semiconductor (CMOS) integratedcircuits, and etch rates are well known and incorporated into commercialSiNx etching apparatus.

The poly-Si layer 613 will then be removed to leave free standingencapsulated nanotube 650, as shown in FIG. 6I. FIG. 6J shows thestructure after an aperture 616 is formed in the encapsulation film 612and insulating layer 608 to expose the metal contact 606. The aperture616 can be formed with standard photolithography and dry etching processthat are known to one skilled in the art.

FIG. 7 is a schematic illustration of an experimental arrangement ofusing such a nanotube sensor as an intracellular probe. FIG. 7 shows twoCNT probes 710, each connected to respective interconnect metal 702formed over substrate 700. The CNT probes 710 are inserted into a cell750 in a liquid bath 720 for electrical probing. A bath electrode 722and the contact pads 704 (provided inside an aperture formed ininsulating layer 708) are each connected to suitable electronics (notshown) to monitor the electrical characteristics indicative ofintracellular activities.

The above embodiments and discussions illustrate the capability tocontrollably deposit a single nanotube with nanoscale lateral precisionnear a center of a region defined by an aperture. The method isparticularly attractive from an implementation or processing viewpoint,because the ability to achieve such controlled deposition within arelatively large region significantly relaxes the requirement forlithographic techniques. As such, the fabrication can readily beperformed using optical lithography, without resorting to morecomplicated lithographic tools (such as e-beam or focused ion beam) toform sufficiently small apertures to define the target depositionregion.

Embodiments of the present invention also provide a method ofcontrolling the number of nanotubes to be deposited and their spacingsin a given region. Such a method is useful for many applications whereit is desirable to deposit more than one nanotube in a defined region.For example, certain vertical field effect transistor (VFET) designs maybenefit from having more than one nanotube forming a channel to allowmore current to flow through the device. Thus, by controlling the numberof nanotubes to be deposited, one can ensure that the VFET output can bedesigned with sufficient current to meet the parameters of a logiccircuit input.

One constraint in the design of the VFET is that the lateral size of thedevice should be as small as possible to maximize the number of VFETsper unit area. One possibility is to fabricate closely spaced vias andconnect each source 802, drain 804, and gate 806 in parallel, as shownin FIG. 8 (with CNT 810 serving as the channel of the device andseparated from the gate 806 by gate dielectric 808). This concept wassuggested by Hoenlein et al., Materials Science and Engineering C, 23,p. 663-669 (2003); and DE 0010036897 C1, (2000). However, the difficultywith fabricating closely spaced vias for positioning nanotubes is thatthe number of nanotubes per unit area is solely determined by theminimum diameter of the vias and the separation between vias. Thisimposes a stringent requirement on lithography and etch processing and,for VFET devices with reasonable maximum current per unit length (1500microampere per micrometer), sub-20 nm diameter vias will be required.

Embodiments of the present invention will allow a device concept such asthat shown in FIG. 8 to be fabricated without imposing stringentrequirements on lithography. Specifically, an aperture can be configuredto control the number of nanotubes, as well as their spacing orpositioning, within the apertured region using electrophoreticdeposition.

FIGS. 9A-B are schematic illustrations of a top view of an apertureconfiguration suitable for controlling nanotube deposition. As shown inFIG. 9A, the aperture has an elongated geometry such as a slot, which ischaracterized by a width (W), also referred to as a lateral ortransverse dimension (along a direction indicated by line X-X′), and alength (L), also referred to as a longitudinal dimension (along adirection indicated by line Y-Y′), with L being larger than W. In thisexample, the width W is designed to be sufficiently narrow so as toallow only one nanotube to be deposited along the transverse direction.Thus, all deposited nanotubes will be deposited in a line pattern, i.e.,lined up adjacent to each other, along the longitudinal direction.

Furthermore, the number of nanotubes deposited within the slot can becontrolled by the length of the slot. Once a first nanotube is depositedin the slot, the electric field distribution around the slot will bemodified. The new field distribution can be calculated using finiteelement analysis. The closest separation between adjacent nanotubes canalso be calculated by using finite element analysis to predict thetrajectory of randomly approaching charged particles that aresuccessively deposited in the slot.

Using this analysis for nanotubes having a length of 100 nm, it has beenestimated that the closest separation between nanotubes with 1 nmdiameter is about 15 nm. For nanotubes with a diameter of 10 nm and alength of 100 nm, the closest separation for adjacent nanotubes is about20 nm. The same method can be used to calculate the closest separationof nanotubes with any geometry. An alternative method can be used tocalculate the electric field in the vicinity of two closely spacednanotubes and reduce the spacing until the calculated electric field hasa distribution that would exclude deposition of a third nanotube inbetween the two that are already deposited.

Once the closest separation (s) between nanotubes is known, the numberof nanotubes, N, deposited in the slot is given by: N=MOD(L/s). Thefunction MOD( ) truncates the resulting number L/s to an integer. Theshape at the ends of the slot may also modify this result, depending onthe degree of rounding. The calculation is most accurate if there is norounding. With the presence of rounding, an additional degree offocusing may reduce the number of deposited nanotubes, and this can bedetermined using three dimensional finite element analysis for the exactgeometry.

As shown above, embodiments of the present invention provide a methodfor controllable depositing nanotubes using electrophoresis in a definedregion. The deposition region may be defined by an aperture, which canbe configured to control the number of nanotubes that can be depositedin the region, as well as the spacings of deposited nanotubes. Byproperly configuring the aperture, e.g., providing a sufficiently smallaperture size such as less than about 100 nm, one can also control thedeposition such that only a single nanotube is deposited in the region,with lateral alignment precision of a few nanometers.

Embodiments of the invention provide a room temperature process that isreadily scalable and compatible with conventional fabrication processesand materials, and allow improved control over the properties ofnanotubes being used in device fabrication.

Although some examples have been discussed in the context of thedeposition of carbon nanotubes, it is understood that the method cangenerally be adapted for deposition of other nanotubes. Furthermore,embodiments of the invention can generally be applied to depositingsingle-walled, multi-walled, semiconducting or metallic nanotubes forfabrication of different devices.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A nanotube-based sensor, comprising: a first insulating layer over afirst conductive layer formed on a substrate, the first insulatinglayer: (i) having a first aperture formed therein exposing a region ofthe first conductive layer, with a deposited nanotube having a first endin contact with the exposed region of the first conductive layer, and(ii) configured to generate an electric field around the first aperture;and a second end of the deposited nanotube coupled to a molecule havinga functional group for interacting with a sample; wherein the firstaperture is sized to allow a plurality of nanotubes to contact theexposed region of the first conductive layer; and wherein after thenanotube is deposited, the deposited nanotube is adapted to re-configurethe electric field to thereby prevent other nanotubes from beingdeposited on the exposed region of the first conductive layer.
 2. Thesensor of claim 1, being one of a chemical sensor or a biosensor.
 3. Thesensor of claim 1, wherein the deposited nanotube is a carbon nanotube.4. The sensor of claim 1, wherein the sensor further comprises a secondinsulating layer substantially surrounding the deposited nanotube, withthe second end of the deposited nanotube not being substantiallysurrounded by the second insulating layer.
 5. The sensor of claim 1,wherein the first aperture has a diameter of at least about 40 nm. 6.The sensor of claim 1, wherein the first aperture has a diameter of atleast about 90 nm.
 7. The sensor of claim 1, wherein the first aperturehas a diameter of at least about 100 nm.
 8. The sensor of claim 1,wherein the first aperture has an aspect ratio of aperture depth toaperture diameter of at least about 0.18.
 9. The sensor of claim 1,wherein the deposited nanotube is an electrophoretically depositednanotube.
 10. The sensor of claim 1, wherein the deposited nanotube is asingle-walled carbon nanotube.
 11. The sensor of claim 1, wherein thefirst end of the deposited nanotube contacts the exposed region of thefirst conductive layer substantially in the center of the exposed regionof the first conductive layer.
 12. The sensor of claim 4, wherein thesecond insulating layer is silicon nitride or PTFE.
 13. The sensor ofclaim 1, wherein the first insulating layer is silicon nitride.
 14. Thesensor of claim 1, wherein the first conductive layer is selected fromthe group consisting of cobalt, nickel and iron.
 15. The sensor of claim1, wherein the substrate is quartz or silicon.
 16. A nanotube-basedsensor, comprising: a first insulating layer over a first conductivelayer formed on a substrate, the first insulating layer: (i) having afirst aperture formed therein exposing a region of the first conductivelayer, with a deposited nanotube having a first end in contact with theexposed region of the first conductive layer, and (ii) configured togenerate an electric field around the first aperture; and a second endof the deposited nanotube coupled to a molecule having a functionalgroup for interacting with a sample; a second insulating layersubstantially surrounding the deposited nanotube, with the second end ofthe deposited nanotube not being substantially surrounded by the secondinsulating layer; a second aperture in the first and second insulatinglayers, the second aperture exposing a region of a second conductivelayer formed on the first conductive layer; wherein the first apertureis sized to allow a plurality of nanotubes to contact the exposed regionof the first conductive layer; and wherein after the nanotube isdeposited, the deposited nanotube is adapted to re-configure theelectric field to thereby prevent other nanotubes from being depositedon the exposed region of the first conductive layer.